docs: add reference for sobol analysis

docs/source/refs.bib

@@ -62,6 +62,17 @@
   author = "Jon Herman and Will Usher"
 }
 
+@article{LeGratiet2014,
+  title = "A {Bayesian} approach for global sensitivity analysis of (multifidelity) computer codes",
+  journal = "SIAM/ASA Journal on Uncertainty Quantification",
+  volume = "2",
+  number = "1",
+  pages = "336--363",
+  year = "2014",
+  doi = "10.1137/130926869",
+  author = "Le Gratiet, Loic and Cannamela, Claire and Iooss, Bertrand"
+}
+
 @article{Mergili2017,
   AUTHOR = {Mergili, M. and Fischer, J. T. and Krenn, J. and Pudasaini, S. P.},
   TITLE = {r.avaflow v1, an advanced open-source computational framework for the propagation and interaction of two-phase mass flows},